To ensure paper quality it is important to know the properties of the wood fibers used in paper making. Important properties include fibre length, fibre thickness and fiber wall thickness. Fiber fibrillation is another important feature. According to prior art these properties can usually be measured by a microscope. In addition, the fiber length can also be measured in an automated manner, in which case this method is suitable for rapidly changing industrial circumstances. The length of a fiber or a fiber-like particle is measured by using a line camera and a thin circular capillary tube. The fibers move one by one in a thin tube from which the line camera forms an image on its detector surface. The fiber length can be formed using the number of pixels covering the fiber image. This method is an efficient way of measuring fiber length, but other fiber properties thus remain unmeasured.